You might want to look at heavy plantings of pampas grass, that grows well beside a natural pond but will not pose anything like a serious invasive problem, long term
Pampas grass is seriously impenetrable, with very sharp saw tooth edges, start out with a number of clumps and you could divide each one four times a year.
It's just as impenetrable in winter, as summer, unless someone burns it down.
Hawthorn tends to grow long and leggy over the years, fairly easy to stoop under leggy stuff like that, it does leave a lot of spiky debris which makes the ground unpleasant to use for all.
Binoculars, night vision goggles and motion detectors may be the only effective remedy to start with for such a large perimeter, if you can cover the approaches to the pond, at least then you will stand some chance of knowing 'when' to be alert... and fret less about intruders...
